Signaling system



fax/ice Aug. 16, 1949 I J, U 2,479,025

SIGNALING SYSTEM Filed Dec. 18, 1945 INVENTOR ef /wss 5' Auao ATTORN EY Patented Aug. 16, 1949 James B. Rudd, Sydney,

New South Wales,

Australia, assignor to Amalgamated Wireless (Australia) Australia, a company of Australia Limited, Sydney, New South Wales,

' New South 'WaleS,

Application December 1 8, 1945, Serial No. 635,792

In Australia This invention relates to signalling systems of the type employing two output circuits coupled to the same output valve, and more particularly to an improved method of combining signal energy from an independent supply source with the signal energy in one of said output circuits without producing undesirable interaction between said twooutput circuits.

The present invention is particularly, though not specifically, adapted for use in aircraft radio receiving installations. In the audio frequency circuits of aircraft re-, ceivers it is frequently desirable to provide two separate output circuits from the output valve of each receiver for the convenience of the first and second pilots. During landing procedures it is often. necessary for one pilot to listen to the outputs of two receivers and for the other pilot to hear the output of one receiver only. The problem of providing this facility without both signals being heard by both pilots has been an exceedingly difficult one to solve. The various methods previously proposed to provide a workable solution of the problem have been summarized by Sandretto in Principles of Aeronautical Radio Engineering, pages 366 to 368. The present invention is an improvement on the system described in connection with Figure 210 of the above reference.

The object of the present invention is to provide an improved method of coupling two output circuits to a common output valve in such a manner that further signal energy from an independent supply source fed to one of said output circuits does not produce cross talk in the other of said output circui The above object is achieved in accordance with the present invention by providing a capacitive coupling between the output circuits of two ultimate output valves.

The invention is more fully described in connection with the accompanying drawing which illustrates a simplified Version of the prior art modified in accordance with the present invention.

In the exemplary arrangement shown in the drawing modulation signal energy, developed across the load resistor R in the output circuit of the amplifying valve VI, is applied to the parallel connected grids GI, G2 of the valves June 15, 1945 4 Claims. (01. 250-20) V2, V3 respectively, across the input resistor RI and through the coupling capacitor C." The anodes of the parallel connected valves V2, V3 are coupled to separate utilization means, which are shown in the present example as telephone headsets A and B, through the output coupling transformers Tl, T2 respectively. v

Operating potentials are applied to the anodes of the valves V2, V3 from the positive terminal HT+ of a potential supply source (not shown) through the respective primary windings of the transformers TI, T2, and to the anode of the valve VI through the anode load resistor R. Biasing potentials for the grids and heating poo-V tentials for the cathodes of the valves Vl, V2 andV3 are derived and applied in any convenient and known manner, 7

Modulation signal energy from the output circuit of an independent signal supply source, such as a separate receiver (not shown), is fed to the utilization device A in any convenient manner through the terminals 3 and 4 in parallel with the modulation signal energy derived from the output of valve V2. With the arrangement thus far described the pilots of an aircraft are enabled, by means of the utilization devices A, B coupled to the separate output valves V2, V3, to listen individually to the signal modulations appearing in the output of the valve VI of a first receiver while the pilot employing the utilization device A is enabled, additionally, to listen to the modulation signal energy appearing in the output circuit of a second receiver (not shown).

Unfortunately, however, signals from the said second receiver, which are applied to the utilization device A through the terminals 3, 4, also reach the utilization device B through a feedback path comprising the grid to anode capacity Cl, the grid resistor RI and the valve V3. One method of attenuating this undesired feedback or cross talk is to reduce the value of the grid resistor RI, but, up to the present, this procedure has required the insertion of an additional stage of amplification before the paralleled amplifiers V2, V3. It has been found that the undesired feedback above referred to may be substantially eliminated if a capacity C2 is connected in accordance with this invention between the anodes of the two output valves V2,V3.

The capacity C2 functions to apply signal potentials, which are fed to the anode of the valve V2 from the second signal supp y source (not shown) through the terminals 3, 4 and the transformer TI, to the anode circuit of the valve V3 in such phase and magnitude as substantially to neutralize the undesired signals from the same source which are fed to the anode of the valve V3 through the feedback path previously de scribed. The magnitude of the undesired signal potentials appearing in the anode circuit of the valve V3 depends upon the values of the grid to anode capacity Cl, the grid input resistor RI, and the mutual conductance of the valve V3.

Provided the reactance of Cl is greater than RI at the highest frequency of operation, and provided that a similar relation holds bet-ween C2 and the load circuit impedance of the valve V3, it is possible to adjust C2 to a value such that alternating currents in the load circuit of V3, due to undesired signal potentials through both paths, are equal in magnitude. For this adjust ment the two currents will not be exactly 180 apart, but they will be so near to this value as to cause considerable attenuation of the unwanted voltage across the load circuit of the valve V3. Best results may be obtained by reducing RI, but it has not been found necessary in practice to reduce Rl to such a low value that an extra stage of amplification is necessary.

Although the required value of C2 may be readily calculated for any particular circuit condition, in oi'derrto facilitate the use or a fixed capacity, the use of a variable capacitor is preferred as it provides a ready means of compensatin'g for any variation in circuit conditions during operation. Although triode valves have been utilized in the exemplary circuit illustrated in the drawing, it is to be understood that pento'des or any other suitable type of valve may be used, if

desired, Without affecting thescope of the invention.

Furthermore, it will also be appreciated that signal energy from the independent supply source (not shown) may be applied to the utilization device B instead of the device A without afifectiiig the operation of an arrangement according to the invention.

In these latter circumstances the capacity C2 functions to apply signal potentials'to the anode circuit of V2 of such phase and magnitude as to substantially neutralize the undesired signals from the same source fed to the anode of V2 through the feedback path comprising the gridanode capacity C3 of the valve V3, the grid re sistor RI and the valve V2.

What I claim is:

1. A system of the type herein specified comprising a first source of signals including an amplifying valve and a signal output load resistor, means for simultaneously applying signal energy, developed across said load resistor in parallel to two separate output circuits, separate utilization devices transformer-coupled to the respective output circuits, a second source of signals, means for additionally applying signal energy from said second source to one of said utilization devices, and a capacitive coupling between said output circuits for attenuating undesired feedback of signal energy from said second source to the other of said utilization devices.

2. In a system for reproducing signals .from a first signal source at a first reproducer, and reproducing signals from said first source and from a second source at a second reproducer: a pair of amplification networks each having an input circuit connected in parallel to the first signal source, and an output circuit separately connected to the different reproducers; signal supply means connected from the second source to only one or" said output circuits; and attenuating means for attenuating the inherent feedback of energy from the second source along the ampli ficatlon networks to the other output circuit, said attenuating means comprising a capacitive coupling between the output circuits.

3. The combination as defined by claim 2 in which each of said amplification networks includes an electron valve-having input electrodes and output electrodes conneotedrespectively-to the input and output circuits; and the attenuating means delivers from one output circuit to the other, signals approximately opposite in phase andequal in amplitude with respect to signals fed back between these circuits by means of the inherent capacitance between the input and output electrodes of the electron valve having its output circuit connected to the second source.

4-. The combination as defined-by claim 3 in which the capacitive coupling is in the form of an adjustable capacitor.

JAMES B. RUDD.

No references cited. 

